Apparatus and methods for providing selectively adjustable blood flow through a vascular graft

ABSTRACT

Apparatus for adjusting blood flow through a graft is provided comprising a balloon disposed against the exterior surface of the graft by a substantially rigid sheath, so that when the balloon is inflated with an inflation medium it causes constriction of blood flow through the graft without creating potentially thrombogenic crimps or infolds in the graft material. The balloon is adjusted by connecting an infusion device to a subcutaneously implanted balloon access port. A pair of ultrasonic elements may be optionally provided to measure blood flow through the graft and to optimize the extent of constriction of the graft. Electrical connection between the ultrasonic elements and external ultrasound processing circuitry is established via a subcutaneously implanted electrical access port that enables multiple electrical connections to be made using a single needle stick.

REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.08/703,096, filed Aug. 26, 1996.

BACKGROUND OF THE INVENTION

Children born with certain congenital heart lesions that includeobstruction of blood flow to the lungs frequently require that a graft,or "shunt" be placed between the systemic arterial system and the lungsto supply blood to the lungs. The first such operation was performed in1945 by Alfred Blalock and Helen Taussig using the native leftsubclavian artery as the graft material. It was a landmark in thetreatment of complex congenital heart disease. Since then, prostheticmaterials, particularly polytetrafluoroethylene (also known as "PTFE" orGortex®, a trademark of W.L. Gore & Associates, Inc., Newark, Del., USA,have replaced harvesting of a native artery as the main graft materialused for this operation. The operation is performed alone or incombination with other procedures in children from the newborn period tothe second decade of life. Such shunts are commonly referred to as"modified Blalock-Taussig shunts."

Despite the current trend toward definitive one stage correction for avariety of congenital cardiac lesions, shunts are still employed inlesions such as tetralogy of Fallot with anomalous coronary anatomy,transposition of the great vessels with ventricular septal defect andleft ventricular outflow obstruction, pulmonary atresia with intactventricular septum, pulmonary atresia with VSD that cannot be closed inone stage, hypoplastic left heart syndrome, complex single ventricleanatomy with pulmonary stenosis, and other lesions in neonates born toosmall to undergo total one stage correction. Additionally, shunts or"fenestrations" between the systemic venous circuit and the pulmonaryvenous atrium are now carried out in a substantial fraction of modifiedFontanu operations performed worldwide.

Together, these anomalies result in the placement of approximately 3000shunts annually in the United States, and another 3000 worldwide. Toinstall a typical modified Blalock-Taussig shunt, the surgeon exposesthe mediastinum (the contents of the middle of the chest between the twolungs) by dividing the sternum. This is the most common access for allheart operations. The pulmonary artery and a major artery (for example,the subclavian artery) are exposed. The distance between them ismeasured, then a segment of PTFE graft of appropriate diameter is cut tothis measured length. The major artery is clamped, and an opening madein the artery. One end of the graft is then sewn to the artery in fluidcommunication with the opening. The pulmonary artery is then clamped,and an opening made in it. The other end of the graft is then sewn tothe pulmonary artery in fluid communication with the opening. The clampsare then released to establish flow in the graft, going from the majorartery to the pulmonary artery (and hence to the lungs).

While performing the operation, the surgeon must make a judgment as tohow large the diameter of the graft should be to provide adequate bloodflow. Unfortunately, the ideal diameter for the graft generally cannotbe predicted with any accuracy. If the graft diameter is too small, thepatient will become too cyanotic (blue) since he or she will not haveadequate oxygen in the bloodstream. Thus, an undersized graft can resultin death in certain cases. If, on the other hand, the graft diameter istoo large, the heart will pump too much blood through the lungs (i.e.,more than is needed), causing the heart to overwork and fail and deathcan result. In both situations the patient can become unstable soonafter operation, and a number of deaths occur each year as a result.Attempts to control a situation of "undercirculation" or"overcirculation" with drugs achieve only modest success.

Periodically, the surgeon must exchange the graft for one of a differentdiameter, usually in an emergency situation requiring a sternotomy inthe Intensive Care Unit because there is insufficient time to reach anoperating room. Every year there are such instances in which the patientdoes not survive this intervention. Even if the patient survives theearly post-operative course, the shunt flow can become inappropriate ata later time, perhaps weeks later, causing heart failure. Additionally,as the patient grows, the shunt flow can become inadequate for thepatient's size. On average, a Blalock-Taussig shunt is used for a periodof days to weeks, and is typically removed at the child's definitiveoperation, which usually occurs within the first twelve to eighteenmonths of the child's life.

In another clinical situation, that of the child with a singleventricle, an operation called a "modified Fontan" is performed, inwhich a conduit is placed between the inferior vena cava and thepulmonary artery. Because the hemodynamic response to this operation issomewhat unpredictable, the surgeon frequently places a secondary shuntbetween the conduit and the common atrial chamber. This shunt is oftencreated using polytetrafluoroethylene material, such as Gortex®, andhence resembles the modified Blalock-Taussig graft. It is frequentlydesirable to regulate the flow through this graft, and eventuallytotally occlude it.

In accordance with previously known methods, the shunt used in themodified Fontan technique has been partially occluded by snaring theshunt with heavy suture material either brought out through the skin, orburied just beneath the skin. In the latter case, when the surgeonwishes to regulate the flow, he incises the skin in a reoperation inwhich the snare is exposed, and tightens down on the snare, thusoccluding the graft. This procedure requires a reoperation each time theflow needs adjustment and is expensive, risky and labor intensive.

The morbidity and cost of the current imprecision in regulating shuntflow in infants is considerable. In a recent case involving aninstitution where approximately 580 pediatric cardiac cases areperformed per year, 10 procedures in which shunts were used orconsidered for use were performed during a three month period. In threecases emergency reoperations were required in the intensive care unitwhile attempting to save the patient by creating or adjusting shuntflow. In three other cases, multi-organ failure and hemodynamicinstability were caused by inappropriate shunt flow.

This experience, in which significant morbidity and cost attended 60% ofshunt-related procedures, highlights the absence in the field ofapparatus and methods for conveniently and accurately adjusting shuntflow. Currently, there are no known "minimally invasive" techniques(i.e., that avoid the necessity of reoperation) for adjusting blood flowthrough these types of shunts.

Devices are known for regulating the flow of blood within nativearteries and fistulas. For example, Edmunds et al. U.S. Pat. No.3,730,186 describes an implantable pulmonary artery band including antoroidal balloon occluder that is disposed around the native pulmonaryartery. The balloon occluder is inflated via a subcutaneously implantedinjection button using a conventional hypodermic needle.

A drawback of the Edmunds et al. device is the lack of a mechanism toaccurately determine the degree of constriction caused by the balloonoccluder. Moreover, the toroidal shape of the balloon occluder isbelieved to create crimps or infolds in the arterial wall even at lowdegrees of constriction. Such crimps or infolds, which project into theflow field of the artery, are expected to disrupt laminar flow withinthe artery and serve as thrombogenic sites.

Lane et al. U.S. Pat. No. 4,828,544, shows a blood control device havinga balloon mounted on a strap which is fastened around a fistula. Theinterior of the balloon is coupled to a pump which is implanted withinthe patient along with the strap and balloon. Actuating the pumpinflates the balloon and restricts the flow of blood through thefistula.

A significant drawback of the device described in the patent to Lane etal. is that it does not permit accurate determination of the degree ofconstriction introduced in the fistula, nor can it provide real-timemeasurement of the flow of blood through the fistula.

In view of the foregoing, it would be desirable to provide implantableapparatus for selectively restricting blood flow through a vasculargraft, so as to provide precise control over the amount of blood flowthrough the graft. It further would be desirable to provide methods andapparatus adapted for connection to the implantable apparatus to provideprecise real-time external measurement and control over the implantableapparatus. It also would be desirable to provide implantable apparatusthat permits constriction of a vascular graft without infolding orcrimping of the graft material, thereby reducing the potential for thedevelopment of turbulent flow.

Subcutaneous ports are known that provide vascular access for patientsneeding chronic intravenous drug administration, such as antibiotics,chemotherapy, or blood transfusions. Such ports are made by severalcompanies, for example, the Infuse-A-Port™ and DualPort™ products madeby Infusaid, Inc., Norwood, Mass., USA, and the Hickman ports made byDavol, Inc., Cranston, R.I., USA. The lowest profile port is theCathLink® 20 by Bard Access Systems, Salt Lake City, Utah, USA.

Access ports permitting electrical connection to an implantable deviceare also known. For example, Soukup et al. U.S. Pat. No. 5,205,286 andModen et al. U.S. Pat. No. 4,941,472 show subcutaneous implants having aplurality of electrical access ports. Each access port accepts a singleelectrical connection. A drawback of the devices described in theforegoing patents is that a separate needle and needle stick is requiredfor each electrical connection, thus increasing patient discomfort andthe risk of infection.

SUMMARY OF THE INVENTION

In view of these and other limitations and disadvantages of previouslyknown devices, it is an object of the present invention to provide animplantable apparatus that selectively regulates blood flow through avascular graft without requiring reoperation.

It is a further object of the present invention to methods and apparatusthat permit optimization of oxygenation and preload to the heart of apatient having a synthetic graft.

It is another object of the present invention to provide apparatus forprecisely measuring the blood flow through a vascular graft inreal-time.

It is a yet further object of the invention to provide apparatus forconstricting a vascular graft in-situ that reduces the potential forcrimping or infolding of the graft material, thus reducing the potentialfor the development of turbulent zones or stagnation points within theflow field of the vascular graft.

It is still another object of the present invention to provideimplantable apparatus for measuring blood flow through a graft, whichimplantable apparatus may be connected to external signal processingapparatus using only a single needle stick.

It is a yet further object of the present invention to provide anelectrical access port which is practical, safe, and comfortable for thepatient.

It is a still further object of the present invention to provide animplantable electrical access port and needle that provides multipleelectrical connections with a single needle stick.

The above and other objects of the present invention preferably areaccomplished by providing an implantable apparatus comprising aselectively inflatable balloon and a sheath for enclosing the balloonagainst an exterior surface of a vascular graft. The balloon is disposedbetween the graft and the sheath so that when the balloon is inflatedwith an inflation medium, it causes constriction of the cross-sectionalarea of the graft without crimping or infolding, thereby reducing bloodflow through the graft while avoiding the creation of potentiallythrombogenic sites. The balloon is maintained in an inflated condition.Optionally, an elastically deformable member may be disposed between thegraft and the balloon to further reduce the development of turbulence inthe flow within the graft at greater degrees of constriction by alteringthe shape of the deformation caused by balloon inflation.

A substantially non-distensible lumen is disposed in fluid communicationwith the interior of the balloon and a balloon access port. The balloonaccess port preferably is implanted subcutaneously within the patient. Anon-coring needle is used to infuse inflation medium into the balloonvia the balloon access port, using a precision infusion device thatallows precise control of the amount of inflation medium introduced intothe balloon.

In an alternative embodiment, a plastically deformable tubular membermay be disposed between the balloon and the exterior surface of thegraft to maintain the graft at a constricted diameter after the balloonhas been deflated.

In accordance with another feature of the present invention, the sheathsurrounding the balloon and vascular graft may in addition have aplurality of piezoelectric elements disposed in opposition across thecross-section of the graft to serve as ultrasound transducers. Thepiezoelectric elements thus provide an ultrasound signal that can becorrelated with blood flow through the graft. In this embodiment of theimplantable apparatus of the present invention, the non-distensiblelumen preferably includes wires that electrically couple thepiezoelectric elements to an electrical access port. The electricalaccess port, also preferably implanted subcutaneously, enables thepiezoelectric elements to be connected to suitable ultrasound activationand signal processing circuitry.

The electrical access port comprises a body having a chamber and aseptum extending across an aperture in the chamber. A plurality ofconductors are disposed resiliently in the chamber to receive acorresponding plurality of conductors disposed circumferentially on theshaft of a needle. The conductors on the needle are coupled to suitablecircuitry for activating the piezoelectric elements to generateultrasound signals and to receive those signals and process them,thereby enabling a physician to monitor the rate of blood flow throughthe graft.

Methods are also provided for selectively adjusting the flow through agraft, and for measuring the flow through the graft, using theabove-described apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention, its nature and various advantages,will be more apparent upon consideration of the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a view, partly perspective and partly schematic, of apparatusconstructed in accordance with the principles of the present invention;

FIG. 2 is a cross-section of the sheath and balloon of FIG. 1 takenalong view line 2--2;

FIG. 3 is a cross section of the apparatus of FIG. 1 taken along viewline 2--2 showing the balloon partly inflated;

FIG. 3A is a cross section view similar to FIG. 3 showing an alternativeembodiment of the apparatus of FIG. 1, wherein an elastically deformablemember is interposed between the graft and the balloon;

FIG. 4 is a longitudinal cross-sectional fragmentary view of theultrasonic element portion of the apparatus of FIG. 1, taken along viewline 4--4;

FIG. 5 is a cross-sectional view of the dual access port portion of theapparatus of FIG. 1, taken along view line 5--5;

FIG. 6 is a top view of an infusion device for use with the dual accessport of FIG. 5;

FIGS. 7 through 9 provide top sectional views of illustrativealternative embodiments of the electrical contacts employed in the dualaccess port of FIG. 5;

FIG. 10 is side view of the distal end of a needle for use with theelectrical access port portion of the dual access port of FIG. 5;

FIG. 11 is a perspective view, partly cut-away, of an alternativeembodiment of the graft of FIG. 1; and

FIG. 12 is a fragmentary sectional view illustrating a step ofre-expanding the graft of FIG. 11 using a balloon dilatation catheter.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1-3, an illustrative embodiment of apparatus 20 forproviding an adjustable flow through a vascular graft, constructed inaccordance with the present invention, is described. Apparatus 20comprises a flexible vascular graft 21 enclosed within sheath 22 withselectively inflatable balloon 23, balloon access port 24 andsubstantially non-distensible connecting lumen 25. Infusion device 100injects and withdraws an inflation medium through balloon access port 24using needle 101, thereby providing a controlled degree of expansion ofballoon 23. Apparatus 20 may optionally include a pair of ultrasonicelements that may be coupled via an electrical access port to ultrasounddriving circuitry 120 and processing circuitry 130, as described indetail hereinafter.

In accordance with the principles of the present invention, inflation ofballoon 23 causes vascular graft 21 to become constricted substantiallywithout crimping or forming infolds that project into the flow fieldwithin the graft. This feature of the present invention is expected toreduce the likelihood of creating potentially thrombogenic sites causedby turbulent flow over the crimped or infolded graft material, or fromstagnation points within the flow field.

Graft 21 comprises a fluid impermeable material, such aspolytetrafluoroethylene (PTFE) or a polyester fabric such as Dacron®, atrademark of E.I. Du Pont de Nemours, Wilmington, Del., USA. Graft 21may optionally include an external winding of a resilient plastic,thereby enhancing elastic recoil of the graft after balloon 23 has beendeflated. Graft 21 is of sufficient length to connect two arteries andof sufficient diameter to provide a range of appropriate constricteddiameters in an intended application. Thus, for example, in a typicalBlalock-Taussig shunt, the length of graft 21 may be about 20 mm, whilea suitable unconstricted diameter may be about 5 mm. As will of coursebe understood, the size of graft 21 depends upon the size of thepatient, and the dimensions of apparatus 20 are preferably tailored tomeet the functional requirements of the particular surgical application.

Sheath 22 preferably is made of plastic (e.g., polyethylene) havingsufficient rigidity to experience little diametral expansion whenballoon is inflated to constrict vascular graft 21. Applicant expectsthat this requirement may be achieved using a sheath having a thicknessof between about 0.3 mm and 1.0 mm, depending upon the materialselected. Sheath 22 preferably is approximately circular incross-section and of a length so that the ends sheath 22 do not impingeupon the suture lines at each end of the graft (where the graft isconnected to the native vasculature). The cross-sectional deformation ofthe sheath should accommodate deformation of the graft during ballooninflation, for example, to take on an elliptical shape to some extent,so as to minimize the amount of space between graft 21 and sheath 22, orbetween sheath 22 and a deformable member (described hereinbelow). Forexample, for a vascular graft about 20 mm long, applicant expects thatthe sheath should extend over approximately the center 10 mm of thegraft.

Sheath 22 may be constructed as two half-circumferences 28 and 29, whichmay be fastened together to enclose balloon 23 against graft 21 usingsuitable biocompatible fastening means, for example, one or moregrommets, clips or hooks. In one embodiment, interior diameter 26 ofsheath 22 is about equal to exterior diameter 27 of graft 21, so thatinterior surface 30 of sheath 22 lies nearly flush with the exterior ofgraft 21, thereby reducing tissue ingrowth between the sheath and thegraft.

In an alternative embodiment, an elastically deformable member may beinterposed between graft 21 and sheath 22, preferably surrounding graft21. The elastically deformable member is constructed to alter the shapeof the deformation of the graft so as to further reduce the developmentof flow stagnation points and the development of turbulent flow areas inthe flow field when the graft is highly constricted. The elasticallydeformable member also serves to restore graft 21 to a larger diameterwhen balloon 23 is deflated. In this case, sheath 22 preferably has anapproximately elliptical shape and may be somewhat deformable toaccommodate deformation of the elastically deformable member, whileminimizing space between the elastically deformable member and sheath 22to reduce tissue ingrowth.

To further reduce tissue ingrowth, which might interfere with operationof apparatus 20, graft 21 and sheath 22 (including the elasticallydeformable tube, if present) may be sealed together. For example, theends of sheath 23 may be sealed to the exterior of graft 21 using a thindrape or wrap of substantially non-porous biocompatible material, suchas PTFE or a silicon-based film, that extends nearly the entire lengthof the graft. Alternatively, graft 21, sheath 22 and balloon 23 may beprovided preassembled (and in a range of sizes) in suitable sterilepackaging directly from a manufacturer. Applicant expects that a drapeof ultra-low porosity PTFE having an internodal distance of about 10 μmwill provide satisfactory protection.

Referring still to FIGS. 1-3, sheath 22 may include raised tube-likeportion 31 arranged parallel to longitudinal axis of the sheath toaccommodate balloon 23 in its deflated condition. Raised portion 31preferably is about 5.0 mm in length and about 1.0 mm in diameter, andpreferably is open along its length to the inside lumen of the sheath.In general, raised portion 31 is shaped so that, when deflated, balloon23 fits within the raised portion and is flush with the interior surface30 of sheath 22. Raised portion 31 may include an aperture near itsmid-length having a diameter between about 0.5 mm and about 1.0 mmthrough which connecting lumen 25 is coupled to provide fluidcommunication between balloon 23 and connecting lumen 25. Alternatively,connecting lumen 25 may be coupled to balloon 23 at either end.

Balloon 23 is made of materials commonly used in balloon catheters, suchas polyethylene, nylon, or similar suitable material, and issubstantially more compliant than sheath 22, so that expansion ofballoon 23 cause constriction of graft 21, rather than diametralexpansion of sheath 22. Balloon 23 may be attached along its length tothe interior surface of raised portion 31 of sheath 22 usingconventional fixation techniques. Balloon 23 is preferably cylindricallyshaped using techniques which are per se known for instilling "memory"in balloon material during manufacturing, for example, usingconventional biaxial orientation techniques. Balloon 23 should have adiameter selected to achieve a maximum desired degree of constriction ofgraft 21 when the balloon is inflated to its maximum diameter. Forexample, the illustrative balloon shown in FIGS. 1-3 may have a maximuminflation diameter of about 6.0 mm and a length of about 6.0 mm. It willof course be understood, however, that because balloon 23 and sheath 22may have to be employed with a graft considerably shorter than 20 mm,balloons of other shapes and sizes may be used, for example, a balloonmay have a maximum circumference of 5 to 6 mm, but a length of only 3mm, so long as substantially no crimping or infolding of the graftmaterial results during balloon inflation.

The compliance of the balloon preferably is such that the inflationpressure does not exceed a so-called "functional value," determined byconsidering the following three factors: (1) the pressure needed todeform the graft when blood is flowing through it; (2) the desire tohave minimal distortion of balloon shape during graft deformation; and(3) the desire to stay within the functional value at which allpressurized components (balloon, connecting lumen, port and allconnections) are leakproof for the desired period of inflation (whichmay vary from days to months, depending on the clinical application).The functional value for most applications is expected to be betweenabout 2 and 3 atmospheres. Such pressures are commonly achieved inpreviously known embolectomy catheter balloons.

The composition and thickness of the balloon material is such that it isessentially leakproof for the selected inflation medium for periodsranging from days to months, depending upon the clinical application.The burst pressure of the balloon material preferably is several timesthe functional value (i.e., higher than that which might be accidentallyachieved during inadvertent aggressive infusion). Balloon 23 preferablydoes not significantly plastically deform or experience significantcreep deformation at its working pressure and over the expected extendedperiods of inflation. The inflation medium preferably also may beevacuated from the balloon after an extended period of inflation rangingfrom days to months, depending on the clinical application.

The inflation medium should be selected to be chemically compatible withthe components of the system, for example, saline or glycerine. While aninert gas may also be used and has the advantage of having a lowreactivity, it is expected that a gas may have a greater tendency toleak.

As balloon 23 is inflated with the inflation medium, it expands so as tosmoothly compress graft 21 without producing kinks, crimps or infolds ineither graft 21 or balloon 23. Graft 21 takes on a fishmouth-shapedcross section that progressively narrows with balloon inflation, asshown in FIG. 3. This deformed shape is expected to maintain laminarflow up to quite small cross-sections. As will of course be understood,laminar flow is preferable to turbulent flow, as the latter may resultin high shear areas which are conducive to pseudointimal hyperplasia, orbuildup of material on the inside surface of the graft. In addition, aregular cross-section is preferable to an irregular one, since the latercan result in complex flow patterns with stagnation points that areconducive to platelet adherence and thrombosis. Slight modifications inthe shape of raised portion 31 of sheath 22 and/or the shape of balloon23 may be preferred in some applications to optimize the graft bloodflow characteristics.

In an embodiment including an elastically deformable support membersurrounding the graft and interposed between the graft and the balloon,inflation of the balloon causes the elastically deformable member todeform to a shape characteristic of that member. For example, such agraft may have a circular shape when the balloon is deflated, but asshown in FIG. 3A, may adopt an elliptical or multi-lobe configurationwhen the balloon is inflated. As will of course be understood, thedeformed configuration of the elastically deformable member 22' shouldbe selected to maintain laminar flow throughout the cross-section of thegraft, without stagnation points, even at the highest degree ofconstriction of the graft.

An estimate of the length scale of geometric irregularities that wouldcause turbulence corresponds to the Reynolds number, Re, for flowthrough the graft, and is computed as:

    Re=v*L*ρ/μ                                          (1)

where:

v=mean velocity;

L=length scale over which geometry varies;

ρ=density of blood; and

μ=dynamic viscosity of blood.

For a neonate with a cardiac output of about 600 ml/min, the Reynoldsnumber is approximately 33. With no graft deformity, assuming L=4.0 mm,the flow is laminar. However, turbulence might be seen when L approaches1.0 mm, which might be the case near the corners of the fish mouth seenin FIG. 3. Accordingly, applicant expects that the use of an elasticallydeformable member as shown in FIG. 3A will provide laminar flow even athigh degrees of constriction of the graft.

Connecting lumen 25 includes a passageway to transport the inflationmedium from balloon access port 24 to balloon 23 and may include anadditional passageway for accommodating a plurality of conductingelements to transmit signals to and from the ultrasonic elements, asdescribed herein below. The length of connecting lumen 25 depends uponthe distance from sheath 22 to the subcutaneous site of balloon accessport 24 and the size of the patient. Generally, in infants, the portsite preferably is subrectus and preperitoneal (behind the rectusmuscle, which is an abdominal muscle), while in older children the sitepreferably is subclavicular. As will be recognized, these sites arecommonly used for pacemaker generators and venous access ports.

Connecting lumen 25 comprises a substantially non-distensible material,i.e., one having a very low compliance. The fluid passageway throughconnecting lumen 25 preferably has an inner diameter between about 2 Fr.and about 4 Fr. As will be readily understood, the use of asubstantially non-distensible material is required so that theconnecting lumen does not "absorb" inflation medium volumes comparableto the balloon volume at working pressures. In addition, connectinglumen 25 should not plastically deform or experience significant creepdeformation over periods of time from days up to months. Likewise, theconnections between connecting lumen 25, sheath 22 and access port 24should not leak over the expected extended periods of inflation.

Referring still to FIGS. 1-3, the flow measurement features of thepresent invention are now described. Two ultrasonic elements 32 and 33are disposed in opposition across graft 21 and electrically coupled tocircuitries 120 and 130 to provide signals indicative of the rate ofblood flow through the graft.

Sheath 22 preferably includes an extended portion 34 that provides anarm upon which ultrasonic element 32 is mounted, while ultrasonicelement 33 is mounted in opposition to, but preferably axially offsetfrom, ultrasonic element 32. Elongated compartment 35 is affixed to theend of extended portion 34 to house ultrasonic element 32; ultrasonicelement 33 is housed in a similar compartment affixed to sheath 22. Thecompartments housing ultrasonic elements 32 and 33 are smoothly-shapedso that they do not erode the surrounding tissue. Elongated compartment35 preferably has dimensions of about 6.6 mm by about 3.0 mm, with thelonger dimension being perpendicular to the longitudinal axis of graft21. Although this angle maximizes the accuracy of the transit-timeultrasonic flow measurement, it is not essential, and may not beachievable for grafts having a relatively small ratio of length towidth.

Ultrasonic elements 32 and 33 preferably are piezoelectric elementscapable of generating an approximately planar acoustic wave. Suitablepiezoelectric elements may be comprise, for example, a series of layersincluding a piezoelectric material, such as copolymers of vinylidenefluoride (VDF) and trifluoroethylene (TrFE), for example, available fromToray Industries, Kamakura, Japan. Use of such materials to formultrasonic transducers is described in Ohigashi et al., "Piezoelectricand Ferroelectric Properties of P(VDF-TrFE) Copolymers And TheirApplication To Ultrasonic Transducers", page 189 et seq., in MEDICALAPPLICATIONS OF PIEZOELECTRIC POLYMERS (Galetti et al. editors), Gordonand Breach Science Publishers S.A. (1988), which is incorporated hereinby reference. Length 36 of the ultrasonic elements are preferablygreater than diameter 27 of graft 21. As illustrated in FIG. 6, extendedportion 34 preferably is dimensioned so that the plane between the facesof ultrasonic elements 32 and 33 form an angle α of about 60° relativeto the longitudinal axis of graft 21.

A pair of thin conducting wires 37, preferably molded within thethickness of sheath 22, connect the ultrasonic elements to conductivewires within connecting lumen 25 and to electrical access port 38. Thewires preferably emanate from sheath 22 at the site at which connectinglumen 25 is attached and are electrically coupled to corresponding wiresextending within a passageway (or attached externally) to connectinglumen 25.

The ultrasonic elements preferably are connected to signal generator 120via electronic access port 38. The signal generator preferably generatesa signal inducing the piezoelectric elements to emit a frequency betweenabout 1.8 MHZ and about 3.6 MHZ, depending on the geometry of theelements. The signal preferably causes an approximately planar wave tobe generated, which is transmitted by element 32 and received by element33 after passing through the blood flowing through graft 21. Thereceived signal is provided to processing circuitry 130 by signalgenerator 120 via the conductive leads associated with connecting lumen25 and electrical access port 38. The processor preferably uses theprinciple of transit time volumetric flow determination to calculate theinstantaneous total flow through the graft. This principle, described,for example, in Drost, C. J., "Vessel Diameter-Independent Volume FlowMeasurements Using Ultrasound," Proc. of San Diego Biomed. Symp., SanDiego, Calif.; San Diego Biomed. Soc., Vol. 17, pp. 299-302 (1978) andU.S. Pat. No. 4,222,407, which are incorporated herein by reference, isused in contemporary volumetric flowmeters for research and clinicaluse, and is considered superior to Doppler techniques, which are flowgeometry dependent.

Applicant expects that a flow resolution of about 2 ml/min can beachieved with the apparatus of the present invention, with a maximumresolved flow of about 5 ml/min and a zero offset of about +15 ml/min,thus providing absolute and relative accuracies of about +15 and +2percent, respectively. Clinical situations typically require a relativeaccuracy of approximately +5 percent.

One notable problem with expanded PTFE graft material is that air maybecome trapped in the interstices of the graft material, which mayscatter the acoustic signal generated by the ultrasonic elements. It hasbeen suggested that this effect may be reduced by pre-treating the graftmaterial with a suitable solvent that can penetrate the interstices ofthe expended PTFE, for example, ethanol, or by lowering the acousticfrequency of the ultrasonic elements, so long as the flow rates throughthe graft are in a range of 500 to 2000 cc/min.

In the illustrative embodiment of FIG. 1, balloon access port 24 andelectrical access port 38 are combined into dual access port 40. As willof course be understood, whether ports 24 and 38 are combined into asingle port depends upon the intended application of the apparatus.Referring now to FIG. 5, dual access port 40 is described in greaterdetail. Dual access port 40 has body 41 which preferably is made ofpolyethersulfone. Polyethersulfone is lighter than titanium (which iscommonly used in subcutaneous port bodies), and is compatible withMagnetic Resonance Imaging scanning.

The balloon access port portion of dual access port 40 includes chamber42 having aperture 43 covered by septum 44. Septum 44 preferably is madeof compressed liquid silicon, so that the septum will not leak evenafter repeated punctures. Smooth, cone-shaped portion 45 tapers fromaperture 43 to lumen 46. Passageway 47 couples lumen 46 to connectinglumen 25.

Referring now to FIG. 6, infusion device 100 for injecting andwithdrawing inflation medium into the balloon access port is described.Infusion device 100 comprises needle 101 coupled to substantiallynon-distensible tubing 102, three-way port 103, syringe 104 removablymounted on support member 105, precision metering system 106, andpressure monitor 107. Syringe 104, needle 101, three-way port 103 andtubing 102 preferably are used only once and disposed, while precisionmetering system 106 and pressure monitor 107 are reusable.

Needle 101 is preferably non-coring, i.e., the needle will not bore outa piece of septum 44 when inserted into balloon access port 24, andpreferably is in a range of 21 to 23 gauge. After needle 101 is insertedinto lumen 46 through septum 44, plunger 108 disposed in syringe 104 ismoved proximally or distally to inject (or withdraw) inflation mediuminto (or from) balloon access port 24 through needle 101. To achieveprecise infusion volumes, the volume of syringe 104 preferably isbetween about 0.5 ml and about 0.75 ml.

Support member 105 of infusion device 100 may include a spring-actionholder 109 for holding syringe 104 securely in place on support member105. In operation, syringe 104 is filled to an appropriate level withinflation medium and secured in holder 109. Serrations 110 on plunger108 are then engaged with teeth 111 of gear 112. Gear 112 forms part ofprecision metering system 106 and is mechanically coupled to calibrateddial 113. The coupling ratio between dial 113 and gear 112 preferably issuch that manual rotation of dial 113 allows precise control over theamount of inflation medium injected into, or withdrawn from, balloon 23via balloon access port 24.

Gear 112 preferably ratchets, to prevent it from inadvertently rotatingin the wrong direction. This feature ensures that transient pressurefluctuations induced in chamber 42 do not cause inflation medium to bepushed back into the syringe. The ratcheting capability of gear 112 mayin addition be reversed to allow rotation of the dial in the reversedirection to withdraw inflation medium from the balloon access port,thereby reducing the balloon volume.

Three way port 103 is attached to the proximal end of syringe 104. Ashort length of low compliance tubing 114 connects an outlet of thethree-way port to pressure transducer 107. Pressure transducer 107continuously measures and displays the pressure in the hydraulic circuitcomprising syringe 104, tubing 102, balloon access port 24, connectinglumen 25 and balloon 23.

Infusion device 100 permits the precise inflation of the balloon, and ispreferable over standard thumb-action plungers, which applicant expectsto be imprecise for use in practicing the invention, since they mayallow a slight return of inflation medium into the syringe as a resultof access port chamber pressure fluctuations. Thus, although notpreferred, it is expected that thumb-action plungers may be successfullyused in certain applications of the present invention.

Referring again to FIG. 5, dual access port 40 also includes a portionhousing an electrical access port having chamber 50. Chamber 50 hassmooth, cone-shaped portion 51 that narrows in diameter from top tobottom along its length. Septum 52, formed from a self-sealing material,such as compressed liquid silicone, extends across aperture 53 inchamber 50. The thickness and resilience of septum 52 preferably aresuch that, after roughly a dozen non-coring needle punctures, it remainsleakproof for periods ranging from days up to months (depending onclinical application).

The diameter of septum 52 and volume of chamber 50 preferably are small,so that the port has a low profile. Thus, for example, chamber 50 maydefine a volume of about 0.1 ml, and is approximately 10 mm in height,with narrowed portion 51 narrowing from a diameter of about 7.0 mm toabout 2.0 mm over a distance of about 2.0 mm. Septum 52 preferably isabout 7.0 mm in diameter and about 2.0 mm thick.

Chamber 50 preferably has lumen 54 containing a plurality of resilientlybiased electrical contacts 55. Spacing between individual ones of theplurality of electrical contacts 55 provide electrical insulationbetween adjacent contacts. In the exemplary embodiment shown in FIG. 5,chamber 50 preferably has a length of about 6.0 mm, while eachelectrical contact 55 is about 0.5 mm wide and separated by a distancebetween about 0.5 mm and about 1.0 mm from neighboring contacts.

Referring now to FIG. 7, electrical contacts 55 may take the form ofsemi-circumferential rings that are biased toward the center of lumen 54by springs 57. Springs 57 may be made of metal or plastic, and mayconstitute small coils rather than U-shaped strips as shown as shown inFIG. 7. Electrical contacts 55 narrow the diameter of the lumen so that,when a small gauge needle (e.g., 22 gauge) constructed in accordancewith the present invention is inserted therein, electrical contacts 55engage mating contacts on the needle.

As will be clear to those skilled in the art, electrical contacts otherthan the two semi-circumferences shown in FIG. 7 may be used inaccordance with the present invention. For example, electrical contacts55 could comprise four quarter-circumferences 55' shown in FIG. 8, or asplit ring 55" shown in FIG. 9. The split ring embodiment of FIG. 9 hasthe added advantage that the spring action is provided by the resilienceof the ring itself, thus obviating the need to provide a spring for eachelectrical contact.

Referring again to FIG. 5, conducting wires 58 are attached toelectrical contacts 55 and coalesce at egress site 59 of dual accessport 40. Conducting wires 58 are enclosed at the exit of dual accessport 40 within connecting lumen 25 so that there is continuousinsulation from the surrounding tissue.

Septum 52 electrically isolates the electrical contacts disposed withinchamber 50 from the environment, prevents leakage of body fluid into thechamber, and wipes all fluid and tissue from the needle (describedherein below) that is inserted into chamber 50 to establish electricalconnection between the ultrasonic elements and the external circuitries120 and 130. To facilitate this wiping function, septum 52 preferably isimpregnated or coated with a hydrophilic substance.

Dual access port 40 preferably is silicone encapsulated and includes aplurality of suture holes located around its base for anchoring body 41to the subcutaneous fascia. The distance between the bottom of chamber50 and the bottom of body 41 preferably is about 2.0 mm. The diameter ofthe base of body 41 preferably is about twice the diameter of the septadiameters 44 and 52, so that dual access port 40 remains with its septafacing outward in vivo. Those skilled in the art will appreciate thatthe shape of dual access port 40 may be varied in accordance with theintended implantation site.

Needle 80, shown in FIG. 10, is constructed in accordance with theprinciples of the present invention to engage and electrically couple toelectrical contacts 55 of electrical access port 38. Needle 80preferably has solid tip 81 including beveled end 82 for smoothlytraversing tissue. Needle 80 has a plurality of circumferentialelectrical contacts 83 separated by insulators 84. Electrical contacts83 mate with respective ones of the plurality of electrical contacts 55disposed within electrical access port 38. Needle 80 is insertedexternally through the skin and into chamber 50 until end 82 contactsthe bottom of the chamber. Needle 80 preferably has a gauge of 20, 21,or 22, although other gauges can be used with the present invention. Itis expected that there will be twice the number of electrical contacts83 as there are ultrasonic elements mounted on sheath 22.

Electrical contacts 83 preferably are about 0.5 mm wide, and separatedby insulators between about 0.5 mm and about 1.0 mm wide, so as to matewith electrical contacts 55 of electrical access port 38. Accordingly,electrical contacts 83 shown in FIG. 10 occupy about 3.5 mm along thelength of the needle. In this embodiment, tip 82 is about 2.5 mm long,so that the tip and electrical contacts occupy about 6.0 mm of thelength of needle 80.

Needle 80 preferably includes hollow core 85 through which passesinsulated conducting elements 86, e.g., wires or thin metal strips. Eachone of electrical contacts 82 preferably is connected to a conductingelement. Conducting elements 86 join to appropriate secondary contacts(not shown) at the end of the needle 80. These secondary contacts areplugged into an appropriate counterpart on the end of a cable to connectthe ultrasonic elements to suitable circuitry 120 for driving theultrasonic elements. Circuitry 120, which is per se known, is connectedto conventional processing circuitry for converting the signals receivedfrom the ultrasonic elements into output representing the flow of bloodthrough graft 21.

In an alternative embodiment (not shown), the access ports may beomitted, and the connecting lumen may be brought out through the skinfor temporary (e.g., approximately one week or less) monitoring and flowadjustment. Such an embodiment is desirable when short term adjustmentsare anticipated, and for use with animal subjects that will beeuthanized for examination. Alternatively, after appropriate preparationof the skin and connecting lumen, the connecting lumen may be cut at theskin level and the remaining length of the connecting lumen allowed toretract under the skin. This latter technique is currently clinicallyaccepted practice for handling temporary epicardial pacing wires placedduring cardiac surgery.

For purposes of illustration, an example of the methods in accordancewith the present invention is now given. First, graft 21, sheath 22,balloon 23, connecting lumen 24 and dual access port 40 described hereinabove with respect to FIG. 1 are implanted within a patient. When thephysician desires to change the flow of blood through the graft 21, thepatient first is connected to EKG and oxygen saturation monitors, andhas temporary IV access established. The skin of the patient then ispierced with needle 80 and it is directed through septum 52 of theelectrical access port portion of dual access port 40 until it is seatedwithin chamber 50. This establishes an electrical connection betweenultrasound elements 32 and 33 and circuitries 120 and 130. Ultrasoundsignal processor circuitries 120 and 130 than confirm a good signalrepresenting the flow of blood through graft 21.

The physician then punctures the skin with needle 101, directing itthrough septum 44 of the balloon access port portion of dual access port40. The physician then infuses or withdraws a small amount of inflationmedium using infusion device 100 to inflate or deflate, respectively,balloon 23, thereby changing the degree of constriction of graft 21 andaltering blood flow through the graft.

The physician carefully monitors the flow signals as well as vital signsand oxygen saturation as the balloon is inflated to establish a desiredflow level. The patient is then observed for about 10 minutes forstability, after which the two needles are withdrawn from the skin. Thepatient is then observed for another short interval, e.g., about 10minutes, prior to termination of the procedure. Advantageously, theforegoing procedure does not have to be performed in an operating room,but can be readily performed in an intensive care unit or a cardiaccatheterization facility.

Referring now to FIGS. 11 and 12, a further alternative embodiment ofthe present invention is described. In FIG. 11, balloon 23 is omittedfrom raised tube portion 31 of sheath 22 for clarity. In thisembodiment, plastically deformable member 90 is inserted between balloon23 and graft 21. Deformable member 90 is preferably a cylinder formed bywrapping a wire mesh around graft 21. When balloon 23 is inflated, itplastically deforms deformable member 90 onto graft 21, constricting thecross-section of graft 21 and altering its flow characteristics. Becausedeformable member 90 is plastically deformed by the inflation of balloon23, it resists re-expansion due to hydraulic forces within graft 21 onceballoon 23 is deflated. Thus, balloon 23, connecting lumen 25 andballoon access port 24 need not be capable of sustaining high pressuresfor prolonged periods, but only for the relatively short intervalsrequired to deform deformable member 90. Deformable member 90 may, forexample, plastically deform between about 0.5 mm and about 5.0 mm in adirection perpendicular to the longitudinal axis of the graft.

As shown in FIG. 12, re-expansion of graft 21 after reduction in thecross-section of deformable member 90 may be accomplished using aconventional balloon dilatation catheter 200 in accordance withwell-known percutaneous catheterization techniques. Thus, expansion ofthe balloon 201 of balloon catheter 200 within graft 21 deformsdeformable member 90 to an expanded diameter, thus increasing flowthrough graft 21. Again, because deformable member 90 is plasticallydeformed by the dilatation catheter, the graft will retain its enlargedcross-section after the balloon dilatation catheter is withdrawn. Theclinical situation in which this deformable stent may be advantageousincludes all cases where it is anticipated that eventual occlusion ofthe graft will be desired, such as pulmonary atresia where there is anintact ventricular septum, and with Fontan fenestrations.

The present invention provides significant advantages over previouslyknown methods of treating children with a variety of complex congenitalheart diseases. However, it is not limited to treatment of children withcongenital cardiac disease. It can be applied to any graft or nativevessel in which measurement and adjustment of blood flow, eitherchronically or acutely in humans or non-humans, is desired.

As also will be apparent to one of skill in the art, balloon access port24 and electrical access port 38 may be used individually in numerousapplications other than those involving grafts. In particular, theability to establish a hydraulic (or pneumatic) coupling to animplantable device, or multiple electrical connections to asubcutaneously implanted access port using a single needle stick,provide significant benefits compared to previously known techniques.The balloon access port and electrical access port and needle of thepresent invention increase the practicality of a number of diagnosticand therapeutic techniques relying on implantable devices.

The balloon access port and inflation apparatus of the present inventionmay be advantageously used in any application where it is desirable tohydraulically or pneumatically activate an implantable device using asubcutaneously implantable access port.

Examples of applications for the electrical access port of the presentinvention include the general category of biosensors, such asion-sensitive field effect transistors ("ISFETS" and "CHEMFETS") usedfor monitoring electrolytes (such as potassium), glucose and pH,clinical applications such as monitoring blood glucose in diabetics,serum potassium in patients with cardiac disease taking multiplediuretics, other electrolytes in renal failure patients, and vaginal pH.Yet other applications include intermittent hemodynamic monitoring usingultrasound technology, such as permanent continuous hemodynamicmonitoring of ventricular function indices in patients with severe heartfailure.

Additionally, direct assessment of central or peripheral neuronalfunction using electrical sensors and stimulators have been proposed andmay be obtained using the electrical access apparatus and methods of thepresent invention. Further, powering and monitoring the function ofpacemakers, implanted cardioverters/defibrillators, and artificialhearts may constitute other important applications of the presentinvention.

Those skilled in the art will appreciate that the invention can bepracticed in other than the described embodiments, which are presentedfor purposes of illustration and not of limitation.

What is claimed is:
 1. Apparatus for for monitoring or controlling animplantable device via a subcutaneous access port, the apparatuscomprising:an electrical access port adapted for subcutaneousimplantation in an animal, the electrical access port comprising:a bodydefining a chamber, an aperture and at least one suture hole; a septumextending across the aperture; a first plurality of electrical contactsdisposed within the chamber; and means for electrically coupling thefirst plurality of electrical contacts to the implantable device; and afirst needle comprising:a cylindrical member; a non-coring tip adaptedto traverse tissue and pierce the septum; and a second plurality ofelectrical contacts disposed on the cylindrical member and adapted toelectrically couple to respective ones of the first plurality ofelectrical contacts; and means for electrically coupling the secondplurality of electrical contacts to external circuitry.
 2. The apparatusas defined in claim 1 wherein each one of the first plurality ofelectrical contacts comprises a resilient member.
 3. The apparatus asdefined in claim 1 wherein the first plurality of electrical contactscomprises a plurality of pairs of semi-circumferential rings, adjacentones of the plurality of pairs of semi-circumferential rings separatedby an insulating circumferential ring.
 4. The apparatus of claim 1further comprising:a fluid access port adapted for subcutaneousimplantation in a living body, the fluid access port comprising:a bodydefining a chamber, an aperture and at least one suture hole; aself-sealing septum extending across the aperture; and a lumeninterposed between the fluid access port and the implantable device andproviding fluid communication therebetween; and infusion meanscomprising:a second needle having a portion defining a bore, and anon-coring tip adapted to traverse tissue and pierce the septum, thenon-coring tip including a portion defining an exit from the bore; andmeans for infusing a fluid into the second access port via the secondneedle.
 5. The apparatus as defined in claim 4 wherein the lumencomprises a substantially non-distensible material.
 6. The apparatus asdefined in claim 5 wherein the fluid actuated portion of the implantabledevice is a balloon.
 7. The apparatus as defined in claim 3 wherein thechamber includes a longitudinal axis and the plurality of pairs ofsemi-circumferential rings are biased towards the longitudinal axis. 8.The apparatus as defined in claim 1 further comprising a plurality ofwires that couple the first plurality of electrical contacts to theimplantable device.
 9. The apparatus as defined in claim 3 wherein thechamber has a cone-shaped portion disposed beneath the septum thatguides the second plurality of electrical contacts into engagement withthe first plurality of electrical contacts.
 10. The apparatus as definedin claim 4 wherein the electrical access port and the fluid access portcomprise first and second portions of a single housing.
 11. Apparatusfor monitoring or controlling an implantable device adapted to becoupled to an external device via a subcutaneous access port, theapparatus comprising:a first access port adapted for subcutaneousimplantation comprising:a body defining a chamber, an aperture and atleast one suture hole; a septum extending across the aperture; a firstplurality of electrical contacts disposed within the chamber, the firstplurality of electrical contacts coupled to the implantable device; anda first needle comprising:an elongated member having a non-coring tipadapted to traverse tissue and pierce the septum of the first accessport; and a second plurality of electrical contacts disposed on theneedle and adapted to electrically couple to respective ones of thefirst plurality of electrical contacts, the second plurality ofelectrical contacts coupled to the external device.
 12. The apparatus asdefined in claim 11 wherein each one of the first plurality ofelectrical contacts comprises a resilient member.
 13. The apparatus asdefined in claim 11 further comprising a plurality of wires that couplethe first plurality of electrical contacts to the implantable device.14. The apparatus as defined in claim 11 wherein the chamber has acone-shaped portion disposed beneath the septum that guides the secondplurality of electrical contacts into engagement with the firstplurality of electrical contacts.
 15. The apparatus as defined in claim11 wherein the first plurality of electrical contacts comprises aplurality of pairs of semi-circumferential rings, adjacent ones of theplurality of pairs of semicircumferential rings separated by aninsulating circumferential ring.
 16. The apparatus as defined in claim15 wherein the chamber defines a longitudinal axis and the plurality ofpairs of semi-circumferential rings are biased towards the longitudinalaxis.
 17. The apparatus of claim 11 further comprising:a second accessport adapted for subcutaneous implantation including:a body defining achamber, an aperture and at least one suture hole; a self-sealing septumextending across the aperture; and a lumen disposed in fluidcommunication between the second access port and the implantable device;and a second needle comprising:an elongated member having a bore and anon-coring tip adapted to traverse tissue and pierce the self-sealingseptum; and means for infusing a fluid into the second access portthrough the bore of the second needle.
 18. The apparatus as defined inclaim 17 wherein the lumen comprises a substantially nondistensiblematerial.
 19. The apparatus as defined in claim 17 wherein theimplantable device comprises a balloon.
 20. The apparatus as defined inclaim 17 wherein the first access port and the second access portcomprise first and second portions of a single housing.